A letter addressed to President Obama containing a substance that initially tested positive for the toxin ricin was intercepted Tuesday by authorities at a remote White House mail screening facility, according to the FBI.

The letter follows the discovery Tuesday of a ricin-laced letter sent to Sen. Roger Wicker (R-Miss.) and came as authorities were investigating suspicious packages in and near the offices of members of Congress.

Edwin Donovan, deputy assistant director of the U.S. Secret Service, did not identify the substance. However, he said the Secret Service is working closely with the U.S. Capitol Police and the FBI.

The substance was detected at a facility run by the Secret Service, Donovan said.

A law enforcement official with knowledge of the matter said the letter to Obama was intercepted in Anacostia, at a facility where all White House mail is initially screened.

It was then sent to a military installation in Maryland where it also failed initial screening for the poison.

Confirmation of ricin can take 24 to 48 hours, depending on the method of testing.

News of the package to the White House came as U.S. Capitol Police were investigating a suspicious package delivered to the front office of Sen. Richard Shelby, an Alabama Republican.

Jonathan Graffeo, a spokesman for Shelby, said in an e-mail that he had no further information.

At midday Wednesday, Capitol police were also investigating a suspicious package left in the atrium of the Hart Senate Office Building.

First-floor offices in the building were evacuated, and people on higher floors were told to stay in their offices.

Authorities later reopened the Hart atrium, without immediately disclosing what — if anything — had been found.

U.S. Capitol Police officials are “talking with someone,” and reports that the man has been arrested are incorrect, aides at the agency said. They would not immediately confirm whether the man had letters in his backpack or say precisely where he was stopped.

“We are currently conducting an investigation,” said Capitol Police spokesman Shennell Antrobus.

The investigation is focused on the Hart and Russell buildings, Antrobus said, declining to provide other information.

Sen. Carl Levin (D-Mich) said in a statement that a staffer received “a suspicious-looking letter” Wednesday morning at the senator’s office in Saginaw, Mich.

“The letter was not opened, and the staffer followed the proper protocols for the situation, including alerting the authorities, who are now investigating,” Levin said. “We do not know yet if the mail presented a threat.”

Letters addressed to the White House are first delivered to a screening facility outside Washington, where they are tested for suspicious substances before being forwarded to a White House mail office located a few blocks from the White House.

Since Obama became president in 2008, the Secret Service has intercepted at least half dozen threatening letters addressed to him, some including suspicious powder or HIV-positive blood.

The White House receives as many as 10,000 letters each week. Protocol demands that every piece of mail be tested for chemical and biological substances and then read by a White House employee.

Each night, 10 letters are forwarded to Obama in a purple folder that is included in his nightly briefing book. By the time a letter reaches his desk, it has usually spent three or four weeks in processing, and it has been read by at least a half dozen White House employees. The arduous mail process, which involves two facilities and 50 employees , was conceived in part to intercept threatening packages such as the latest one.

In 2004, three Senate office buildings were closed after preliminary tests found ricin delivered through the mail system in the Senate majority leader’s office.

At the time, the Associated Press wrote, “Twice as deadly as cobra venom, ricin, which is derived from the castor bean plant, is relatively easily made and can be inhaled, ingested or injected.” But investigators later said the test may have picked up non-toxic byproducts of the castor bean plant used in paper production.

Several letters with white powder were delivered to Capitol Hill during the 2001 anthrax scare, and lawmakers, Obama and Cabinet secretaries remain a target of copycat attacks.